Liquid crystal display device has widely been applied to monitors of personal computer and portable device, by virtue of its advantages including low voltage operation, low power consumption, and potential of downsizing or thinning. In particular, the liquid crystal display device for use in television set with a large screen size, which is assumed to be viewed from a variety of angles, is under strict demands for viewing angle dependence. In recent years, demands for higher levels of performance have also been directed to a liquid crystal display device for monitor use. To cope with the demands, there have been investigated and proposed a variety of operational modes reduced in the viewing angle dependence, based on optimized alignment of liquid crystals in a liquid crystal cell, including IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode, and VA (Vertically Aligned) mode.
However, the viewing angle characteristics have not fully been satisfied yet even with these liquid crystal display devices. Efforts for further improvement have yielded the liquid crystal display devices at present, basically configured by a liquid crystal cell, a phase difference film (also referred to as “phase difference plate”) for improving the viewing angle characteristics, and a polarizing plate.
The phase difference film is used for canceling coloring of image or for expanding the viewing angle. Known techniques regarding the phase difference film include use of a resin film stretched to induce therein birefringence and attached to a polarizing plate, or provision of a liquid crystal layer having liquid crystal molecules aligned therein to an isotropic protective film for a polarizing plate, so as to give birefringence which allows the film to serve as a phase difference layer.
These techniques are, however, in need of providing the phase difference film or phase difference layer in addition to the protective film for a polarizing plate which raises problems in complicating a method of manufacturing the polarizing plate and pushing up to the cost.
In contrast, there has been another proposal of a phase difference film which enables manufacture of a polarizing plate capable of improving the viewing angle characteristics with a simple configuration by giving a function of the phase difference film to a cellulose ester film having widely been used as a protective film for polarizing plate (see Patent Literatures 1 and 2, fore example).
Patent Literature 1 discloses a technique of expanding the viewing angle of a VA-mode liquid crystal cell by applying thereto a phase difference film obtained by width-wisely stretching a film composed of a mixed fatty acid ester of cellulose which is excellent in the stretchability. Patent Literature 2 describes a phase difference film given a desired level of phase difference value by adding a retardation enhancer to cellulose triacetate.
However, in the phase difference film described in Patent Literature 1, obtained by stretching the film composed of the mixed fatty acid ester of cellulose, the potency of expression of birefringence will largely be affected by the degree of acyl substitution of the cellulose ester resin. It is therefore necessary to strictly control the degree of acyl substitution. In order to strictly control the degree of substitution of the mixed fatty acid ester of cellulose (cellulose acetate propionate, for example), the degree of substitution of the acetyl group and the propionyl group have to be controlled in a strict manner. This has, however, pushed up the manufacturing cost, and has limited cost reduction of the phase difference film.
On the other hand, the optical film described in Patent Literature 2, obtained by adding an additive such as retardation enhancer aiming at imparting the optical film with a function of the phase difference film, will unfortunately increase forward scattering (haze), and will consequently degrade the axial contrast of a liquid crystal display device such as VA-mode liquid crystal display device characterized by its very large front contrast since the additive will behave as a foreign substance.
Techniques for reducing such problems in forward scattering (haze) ascribable to the addition of retardation enhancer, and bleeding out of the additive have been proposed typically in Patent Literature 3, proposing use of the additive such as retardation enhancer and a cellulose ester resin having solubility parameters (SP values) close to each other. More specifically, it proposes to adjust the difference of the SP values of the additive and the cellulose ester resin to 1.5 or smaller. The cellulose ester resin used herein has an average degree of substitution of acetyl group of 2.0 to 2.6 by which an excellent stretchability is ensured.
The present inventors, however, found out from further investigations that a recent VA-mode liquid crystal display device, which has a very large front contrast ratio such as 5000:1, caused problems of lowered front contrast and nonuniform contrast when the polarizing plate was manufactured by using an optical film obtained by the technique disclosed in Patent Literature 3 on one surface of a polarizer and the polarizing plate was used for the liquid crystal cell of the device.